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When President Barack Obama signed the US health-care reform act into law on 23 March, he seemed to have emerged victorious from a bitter and protracted political battle. But the fight over the legislation promises to continue beyond the midterm elections next month. If the Republicans gain a significant lead in the House of Representatives, "there will be political trench warfare", says Henry Aaron, a senior fellow at the Brookings Institution, a think tank based in Washington DC.

Health insurance in the United States is provided mainly by the private sector, and 16.7% of Americans don't have any. The reform act aims to cut the number of uninsured and to stem soaring costs — two factors that threaten to cripple the US health-care system.

In addition to requiring that everyone take out health insurance, the act gives the makers of complex biologic drugs 12 years free from generic competition, and offers a tax credit to young biotechnology firms. It also sets up the Patient-Centered Outcomes Research Institute (PCORI) to coordinate comparative-effectiveness research, which compares treatment options to see which is the most effective.

But comparative-effectiveness research has proved controversial. Proponents say it will cut wasteful spending, but critics claim that it means health-care rationing and even, in an erroneous assertion, the formation of 'death panels' that limit access to expensive but life-saving treatments. Debate over the research, which has largely died down while Republicans focus on attacking insurance provisions in the health-care act, could resurface in the spring, says Dan Holler, spokesman for the conservative advocacy group Heritage Action for America, based in Washington DC.

Some Republicans boast that they will try to repeal the health-care act — a bold strategy that is unlikely to succeed. Any such attempt would presumably provoke a veto from Obama, and it is doubtful whether the Republicans could amass the two-thirds majority needed to override a presidential veto.

Another approach is to underfund specific programmes. Representative Michael Burgess (Republican, Texas), the ranking Republican on the Subcommittee on Oversight and Investigations of the House Committee on Energy and Commerce, has already pledged to take this approach. PCORI is one provision that could be vulnerable. Funding has been set aside for it to 2012; thereafter, its money will come from sources including $150 million in appropriations and a $2 fee tacked onto insurance plans. But Republicans could attack PCORI by restricting funds to the agency responsible for collecting the $2 fee. "You certainly could see a significant spending fight on comparative effectiveness," says Holler.

Meanwhile, voters in Arizona (see 'State watch: Arizona'), Colorado and Oklahoma will decide whether to amend their state constitutions to prohibit any requirement that citizens take out health insurance. In August, Missouri voters overwhelmingly passed similar legislation. Although national law would trump these state mandates, making them largely symbolic, 20 states have launched a lawsuit asserting that the health-care act violates the US constitution.